1. Field of the Invention
The present invention relates to vibrational gyroscopes, and more particularly, to high performance stemless hemispherical resonator gyroscopes.
2. Background Art
Generally, the present invention is related to coriolis vibration gyroscopes (CVGs) that typically use resonators made of quartz. Such gyroscopes are described, for example, in U.S. Pat. Nos. 4,951,508, 4,157,041, 3,719,074, 3,656,354, 6,357,296 and 5,383,362.
Numerous geometries of vibrational structures are known, and these various geometries are commonly used in vibrational gyroscopes. For example, such structures include disks, rods, cylinders, hemispheres, etc. The vibrating elements can be made out of different materials, such as ceramics, glass, quartz, metal, although the use of quartz or fused silica is most common. Usually, the best performance is provided by gyroscopes whose resonators have a high degree of axial symmetry, and the resonator is made of a high-Q material. Since fused silica possesses such characteristics as high degree of stability of elastic characteristics, and since a hemisphere has the highest degree of axial symmetry of all the possible resonator geometries that are commonly used, gyroscopes that use hemispherical quartz resonators tend to have the highest precision. The Q factor of many such resonators can reach several million, while metal resonators rarely have a Q factor higher than a few tens of thousands.
A CVG can also function as an angular velocity sensor that detects rotation in two possible modes of operation—an open-loop mode, and a closed-loop mode. The closed-loop mode is also sometimes referred to as a force-rebalance mode. The CVG can also function as an integrating gyroscope, also known as a “whole-angle mode,” which measures the angle of rotation of the gyroscope. See D. D. Lynch “Standard Specification Format Guide and Test Procedure for Coriolis Vibratory Gyros,” September 1998 meeting of the IEEE GAP in Stuttgart, Sep. 18-19, 1998.
In the simplest mode of operation, the open-loop mode, a standing wave is excited in the resonator in one of its modes of vibration (the drive mode). Usually, the second vibration mode is used, with an amplitude that is maintained constant by an automatic gain control system (AGC). At the same time, a standing wave is produced within the resonator, which, in the second vibration mode of the resonator, has four nodes and four anti-nodes. When the entire vibrating structure rotates about its axis, a coriolis force results, given by the equation Fc=2[Ω×V], where Fc is the coriolis force, Ω is the angular velocity of the resonator about its axis symmetry, and V is the linear velocity (in the radial direction, to and from the cylinder center axis). The coriolis force Fc generates vibrations in the sense mode, which are measured, and whose amplitude is proportional to the angular velocity Ω. The spatial orientation of the two modes is 45 degrees relative to each other, for the second vibration mode.
When the CVG works in the open-loop mode, its bandwidth is directly related to the Q factor of the coriolis vibration mode, in other words, to the damping time constant of the coriolis vibration mode. When the Q factor is relatively high, for example, Q=10000, the bandwidth of the resonator is on the order of Δf=(πfc)/Q≈1.5 Hz if the frequency of excitation of the resonator fc=5000 Hz. Such a gyroscope can, in practice, only measure relatively constant angular velocities. Such measurements are usually done, for example, using gyrotheodolite (a gyro-optical instrument used to measure the azimuth fixed by a theodolite direction) when measuring the azimuth of a given direction.
To increase the bandwidth of the gyroscope, it is necessary to ensure that the coriolis mode of vibration damps down relatively quickly, which in turn leads to a lower Q factor of the measured vibration mode, and, consequently, to an increase in the gyroscope's bandwidth. The damping down of the measured vibration mode is done in the closed-loop mode, in other words, in the force rebalance mode. In this mode, the nodal point signal is measured, which is the same thing as the sense mode signal, and a negative feedback signal is generated, which compensates for the signal arising in the nodes by supplying an anti-phase signal to one of the free nodes or to two diametrically opposite nodes out of the four nodes. Therefore, the measured mode of vibration is also suppressed, leading to a relatively low Q factor. With a Q factor of 100, the bandwidth would be approximately Δf=150 Hz. A CVG with such a bandwidth can be used in many inertial systems that are mounted on moving objects.
In the whole-angle mode, the Coriolis force Fc that results from the rotation of the resonator converts the energy of the vibration from the sense mode into the excitation mode and back, where the superposition of these two modes can be measured. Also, in this case, the standing wave in the resonator rotates together with the resonator. The angle of rotation of the standing wave lags behind the angle of rotation of the gyroscope by a constant factor, which is defined only by the working vibration mode. For the second mode of vibration, the constant factor is approximately 0.32, for the third mode of vibration, the constant factor is approximately 0.25.
The design of the gyroscope that uses a hemispherical resonator, as described above, suffers from a number of problems. One of these problems is the difficulty in mass-producing a relatively complex-shaped part—the meniscus-shaped resonator with a stem, which is used for mounting. Such a shape is relatively difficult to produce in mass quantities. Another problem is that due to the complex shape, maintaining perfect axial symmetry of the resonator is extremely difficult. Typically, during manufacture, the body of the resonator (the hemispherical portion) has thickness mismatches, which require extensive rebalancing and/or micro-machining to eliminate. This raises the cost of the resonator considerably, and increases the manufacturing time.
Another difficulty with such conventional gyroscopes is the need to use capacitors for generating and detecting the vibration modes of the resonator. These capacitors typically require a relatively high voltage, on the order of several hundred volts, at times as much as 600 volts. Such high voltages are very inconvenient to work with, particularly where the overall device itself needs to be small. Also, the use of such high voltages tends to result in a shorter life span of the device, and a faster wear on the electrical components of the device. Note that the disadvantages described above apply to both the open-loop and the closed-loop gyroscopes. Another problem is that due to the high voltages involved, the power consumption of the device tends to be substantial.
Accordingly, there is a need in the art for a high precision vibration gyroscope that addresses some or all of these problems.